You wrote:
clare at snyder.on.ca wrote:

Vent free heaters produce COPIOUS amounts of water vapour...

Exactly 2.2 pounds per hour, with 20K Btu/h of heat.

That would depend on the Relative Humidity of the area you live in....

No, but removing it does... 37 cfm of fresh air will keep the indoor RH at
50% at 70F on an average 31.8 F December day in Allentown, PA. In a perfectly
airtight house (vs an average US house that naturally leaks 224 cfm), we
might do that with a humidistat and a muffin fan, or open a 2'wide x 3'tall
window with an A ft^2 gap at the top and bottom with A = cfm/(16.6sqrt(HdT))
= 37/(16.6sqrt(3'(70-31.8)) = 0.21 ft^2, ie a 2'wide x 1.25" gap at the top
and bottom. In a non-airtight house with a window 24' below a leaky upstairs,
the gap might be 37/(16.6sqrt(24'(70-31.8)) = 0.074 ft^2, ie a single 0.44"
max gap at the bottom of a first-floor window.

Nick